controllers/owners: split owner remove query to remove by username and remove by team name

[moskirathe]

This PR is a partial extraction from #14213.

It splits the owner_remove query to owner_remove_by_username and owner_remove_by_team_name. This will allow us to change the behaviour of ower_remove_by_username so that we can compare usernames names using the canon_username function (which also normalizes hyphen to underscore), but keep comparing team names using lower - github allows separate team names that only differ by underscore and hyphens to exist.

Tests:
Owner remove is now routed through Login:parse, which rejects malformed logins before any DB lookup happens. Login::parse currently treats any login containing : as a team login and hands it to GitHubTeamLogin::parse, which rejects malformed ones with a bad_request.

Previously remove_owners passed the raw string straight to krate.owner_remove, which did the lookup, matched nothing (stored logins never contain :), and returned OwnerRemoveError::not_found(login).

So the tests, which assert the old fall-through behavior, now get a parse error instead.